Alani The Stormborn is the new TLPD, she flies around vale of eternal blossoms and drops Reins of the Thundering Ruby Cloud Serpent.

Here's a guide from wowhead about her^

Mount Drop: This rare spawn drops the mount Reins of the Thundering Ruby Cloud Serpent... however, it is unattackable while its protective buff, Nimbus Shroud, is still active.

To Remove the Protect Buff:
To remove the protective buff Nimbus Shroud from Alani, you need a Sky Crystal and it is created from combining 10x Skyshard. This is where things get tough though. If you have not noticed, Skyshard and Sky Crystal are BoP. AND... unfortunately... they are super rare drops from all MoP NPC's that are level 90. However, if you collect 10 of these crystal fragments, you can combine them into a Sky Crystal and fight this rare dragon spawn.

How to Collect Shards:

Option 1 -Level 90 creatures in MoP. They appear to have a much better drop chance from creatures in Vale of Eternal Blossoms, which makes sense if you listen to the backstory behind the rare spawn from Loh-Ki in the zone.
Option 2 - Daily quests that reward Treasures of the Vale have a chance to drop the shards as well. Check out the items page for drop chance % as more people do the dailys.

Fighting this Rare Spawn:
Blizzard seems to have been smart with this rare spawn. When you use your Sky Crystal on the dragon, you channel about 20% of the dragons health away and tag it for yourself. So the kill cant be ninja'ed from you. In addition, Its health is SUPER low compared to all the other MoP rares added. At level 90, you will only need mostly Heroic dungeon gear or the first tier of PVP gear to kill it without much problem. At level 90, this is easily done. Seems gathering the crystals is the only challenge in getting this rare spawn killed.

Rarity of this Spawn:
-(edit: clarified since people seem to not understand what I meant by "spawns very, very often")-
This Rare Spawn is unique from all other mount dropping rares in game so far. When it spawns, it flies around the entire Vale of Eternal Blossoms zone and is very, very visible. It has a despawn timer... but respawns often when it despawned due to not being killed (as of Beta 9/18/2012). Respawn timer from being killed is still being determined. So while it is a rare spawn, the real rare thing about this mount is that very few people will be lucky enough to farm enough of the crystals to attack it.